Define kernel-vanilla as source variant
    
    The vanilla_only macro is overloaded. It is used for determining if
    there should be two kernel sources built as well as for the purpose of
    determmioning if vanilla kernel should be used for kernel-obs-build.
    
    While the former can be determined at build time the latter needs to be
    baked into the spec file template. Separate the two while also making
    the latter more generic.
    
    $build_dtbs is enabled on every single rt and azure branch since 15.3
    when the setting was introduced, gate on the new $obs_build_variant
    setting as well.
    
    Signed-off-by: Michal Suchanek <msuchanek@suse.de>